A multilayer ceramic capacitor basically comprises a stack consisting of a plurality of dielectric members formed of a ceramic material, with electrodes positioned between the members. The electrodes may be screen-printed onto the ceramic material, in the unfired state thereof, using conductive inks. A stack of screen-printed dielectric members is assembled, pressed together, cut into individual components, if appropriate, and fired until sintering occurs, in order to reduce non-porosity.
With the originally employed dielectrics the capacitors had to be fired at temperatures of the order of 1200.degree.-1400.degree. C., which meant that the internal electrodes had to be of a suitable material to withstand such temperatures and that, therefore, expensive noble metals, such as platinum or palladium, had to be used. However, by suitable choice of the dielectric it is possible to reduce the firing temperature thus enabling the use of internal electrodes with a high silver content (50-100% silver), which reduces the cost of materials and manufacture. A dielectric composition which can be fired at a temperature between 950.degree. C. and 1100.degree. C. and can thus be used with high silver content electrodes is disclosed in our GB Patent Specification Serial No. 2107300B (J. M. Wheeler 1). The compositions disclosed therein comprise non-stoichiometric lead magnesium niobate (PbMg.sub.1/2 Nb.sub.1/2 O.sub.3) with one or more of the following, namely lead titanate, lead stannate, lead zirconate. Some of these compositions have dielectric constants in the range 7500-10,000 which makes them particularly suitable for multilayer ceramic capacitors. The originally employed ceramics (U.S. coding Z5U) were not compatible with high silver content electrodes and usually had dielectric constants lower than 7500-10,000.
The electronics industry generally requires smaller components and smaller and cheaper capacitors can be obtained by producing dielectrics which have low firing temperatures so that they are compatible with high silver content electrodes and having even higher dielectric constants than those mentioned above. One such dielectric composition is disclosed in our GB Application No. 8405677 (Serial No. 2137187A) (J. M. Wheeler--D. A. Jackson 3-1X) and is based on non-stoichiometric lead magnesium niobate together with non-stoichiometric lead zinc niobate. This dielectric composition may also include one or more simple oxide additives chosen from silica, manganese dioxide, zinc oxide, nickel oxide, alumina, ceric oxide, lanthanium oxide, tungsten oxide, gallium oxide, titanium oxide and lead oxide. One or more of the following complex oxide additives may also be added to the basic composition, bismuth stannate, bismuth titanate, lead stannate, lead zirconate and lead titanate with or without a simple oxide additive. Such compositions fire at temperatures between 980.degree. C. and 1075.degree. C., have dielectric constants at 25.degree. C. in the range of 9000 to 17,600 with Z5U temperature coefficient of capacitance characteristics and generally low tan .delta. (%) (dielectric loss) at 25.degree. C.
It should be noted that the lead magnesium niobate employed in the above dielectric composition is non-stoichiometric and is not the conventional stoichiometric PbMg.sub.1/3 Nb.sub.2/3 O.sub.3. In Patent Specification No. 2107300B the expression PbMg.sub.1/2 Nb.sub.1/2 O.sub.3 was employed to distinguish from the conventional PbMg.sub.1/3 Nb.sub.2/3 O.sub.3. The material employed for the results quoted in the above specifications is in fact PbMg.sub.0.443 Nb.sub.0.5001 O.sub.3, which approximates to PbMg.sub.1/2 Nb.sub.1/2 O.sub.3. Preferably the magnesium was in the range of 0.35 to 0.5 and the niobium was in the range 0.4 to 0.8 and thus the lead magnesium niobate was non-stoichiometric.